1. Field of the Invention
The present invention relates to a scanning optical system which performs optical scanning so as to form an image in an observer's eye or on a screen and can be observed and which is suitable for an image display apparatus, such as a head-mounted display (HMD) or a projector.
2. Description of the Related Art
Generally, an image display apparatus, such as a head-mounted display (HMD) or a projector, uses a transmission type liquid crystal, a reflection type liquid crystal, or an EL element as an original-image display element. However, recently, an image display apparatus which displays a two-dimensional image by using a light-beam-scanning means (see U.S. Pat. No. 5,467,104) has been proposed.
U.S. Pat. No. 5,467,104 discloses a scan type image display apparatus which scans beams of light of R, G, and B in a horizontal direction and in a vertical direction and forms an image directly on the retina of an observer through an optical system.
However, in the scan type image display apparatus disclosed in U.S. Pat. No. 5,467,104, optical scanning is required to be performed at very high speed, and therefore a very small device is used as an optical scanning means such as a mirror. Therefore, the scanned light beam becomes extremely thin, and the light beam at the position of the pupil of the observer also becomes very small in diameter, thus making it difficult to obtain an exit pupil having a sufficient diameter.
Techniques disclosed in U.S. Pat. No. 5,701,132 and in U.S. Pat. No. 5,757,544 can be mentioned as a method for enlarging such an exit pupil having a small diameter. In U.S. Pat. No. 5,701,132, the expansion angle of a light beam which has passed through an exit-pupil-enlarging means, such as a lens array or a diffusing plate, is increased by disposing the exit-pupil-enlarging means on an intermediate image plane having a curvature formed by a scanned light beam and by allowing the light beam to pass therethrough.
Likewise, in U.S. Pat. No. 5,757,544, an exit pupil is enlarged by allowing light made incident to an image display element, such as a liquid crystal illuminated with parallel rays of light, to pass through a diffusing plate, although the apparatus disclosed in this patent is not a scan type image display apparatus.
However, since the apparatuses disclosed in U.S. Pat. No. 5,701,132 and in U.S. Pat. No. 5,757,544 each use the exit pupil enlarging means of the intermediate image transmission type, an optical path becomes long, and the apparatus is easily enlarged in size.
Additionally, Japanese Patent Application Laid-open No. 2001-4955 proposes a relatively small-sized optical system in which light from a light source is changed into a slightly thick beam of light by using a decentration-free curved-surface optical system, and the light beam is guided to an eyeball while being reflected by a scanning means. In this publication, there is mention of an exit pupil being enlarged by a transmission-type diffusing plate, however, the system is inevitably enlarged in size, resulting from use of the transmission-type diffusing plate. In other words, in this case, the system can only be reduced to have approximately twice the size of a free curved surface prism known as representative of a small ocular optical system (which is disclosed in Japanese Patent No. 2911750 and is described in “Optics,” Vol. 25, No. 1, pp. 2-7, 1996).